Engine harmonic cancellation (EHC) systems are active noise reduction systems that are used in motor vehicles, for example in cabins or in muffler assemblies, to reduce or cancel engine harmonic noise. EHC systems use one or more microphones as input transducers. A signal related to the noise to be canceled is also inputted to an adaptive filter. The output of the adaptive filter is applied to one or more transducers that produce sound (i.e., loudspeakers). The sound is acoustically opposite to the undesirable engine sounds that are to be canceled. The adaptive filter can alter the magnitude and/or the phase of the input signal. The aim of the system is to cancel the microphone signal at the frequency or frequencies of the sinusoidal engine noise by using the sound transducers to output sinusoids of the same frequencies and amplitudes but opposite (180 degree offset) phase.
In certain situations these EHC systems can cause the loudspeaker sound output levels that are designed to cancel the engine noise to be greater than the level of the noise to be cancelled. This can cause an audible noise artifact (also called “afterglow”), which is undesirable. Afterglow can occur when there is a sudden decrease in the engine load (e.g., when there is a transmission up shift or down shift) and a resulting sudden decrease in the engine noise level in the cabin that occurs while the EHC output briefly remains at the sound pressure level it was at before the noise decreased. The EHC system must readapt to the new, lower engine noise level to resume its noise cancellation and this process is often slower than necessary to avoid a temporary noise gain.